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A photograph sits amid the tributes outside the home where a pregnant woman and her two daughters lived Thursday, Aug. 16, 2018, in Frederick, Colo.

The bodies of two young girls were submerged in crude oil for four days before authorities discovered them on Thursday, according to court documents filed Friday by an attorney defending the girls' father against accusations that he killed his children and wife.

The motion filed Friday by Christopher Watts' attorney, James Merson, also asked that DNA swabs be taken from the girls' necks. The request quotes an expert who believes the oil would not eliminate DNA and said samples can be obtained "after strangulation."

Authorities separately announced that the Weld County Coroner's Office had performed autopsies on Friday and confirmed the bodies as 34-year-old Shanann, 4-year-old Bella and 3-year-old Celeste Watts.

Police did not release any information about how the mother and daughters died. More testing is planned to help determine the cause of their deaths.

Richard Eikelenboom, the expert cited by Watts' attorney, also recommended taking DNA samples from the girls' hands and the hands and nails of their mother. Eikelenboom has testified in several high-profile criminal trials, often on so-called "touch DNA" when small samples of genetic material are left on a surface.

After his wife and daughters were reported missing on Monday and before his arrest, Watts told reporters he missed them and longed for the simple things like telling his girls to eat their dinner and gazing at them as they curled up to watch cartoons.

Authorities are expected to file formal charges Monday against Watts, an oil and gas worker who authorities said dumped his wife and daughters' bodies on his employer's property.

Police said the mother, Shanann, was found dead on property owned by Anadarko Petroleum, one of the state's largest oil and gas drillers, where 33-year-old Christopher Watts worked as an operator. Investigators found the bodies of Bella and Celeste nearby.

Watts was fired on Wednesday, the same day he was arrested, the company said. He did not respond to reporters' questions when he was escorted into the courtroom Thursday.

Merson, Watts' attorney through the Colorado State Public Defender's Office, left Thursday's court hearing without commenting to reporters. He did not respond to multiple messages seeking comment by The Associated Press.

Police have not released any information about a motive or how the three were killed.

The family's two-story home is just outside Frederick, a small town on the grassy plains north of Denver, where fast-growing subdivisions intermingle with drilling rigs and oil wells.

According to a June 2015 bankruptcy filing, Christopher Watts had gotten a job six months earlier as an operator for Anadarko, and paystubs indicate his annual salary was about $61,500. Shanann Watts was working in a call center at a children's hospital at the time, earning about $18 an hour — more for evenings, weekends or extra shifts she sometimes worked.

But the family remained caught between a promising future and financial strain from debt and other obligations.

The couple had a combined income of $90,000 in 2014. But they also had tens of thousands of dollars in credit card debt, along with some student loans and medical bills — for a total of $70,000 in unsecured claims on top of a sizable mortgage.

They said in the filing that their nearly $3,000 mortgage and $600 in monthly car payments formed the bulk of their $4,900 in monthly expenses.

Details about what led police to arrest Watts late Wednesday night on suspicion of three counts of murder and tampering with evidence likely were to be revealed next week.

Prosecutors will ask a court to unseal the affidavit for Watts' arrest after filing formal charges, which are due by mid-afternoon on Monday, said Krista Henery, a spokeswoman for district attorney Michael Rourke.

Family and friends of Shanann Watts are left searching for answers, trying to reconcile Watts' cheery Facebook posts about her daughters, her pregnancy and her love for her husband with the pending charges.

Ashley Bell met Shanann Watts about two years ago, when the mother of two came into Bell's new tanning salon in nearby Dacono. The two women quickly became friends, and before long they were texting or calling each other almost daily. Their daughters played together during salon visits.

"I just don't understand it," said Bell, who described Christopher Watts as a loving father.

Shanann Watts was from North Carolina, and her parents' next-door neighbor, Joe Beach, said he saw her recently when she visited the neighborhood of modest homes in Aberdeen.

"We were talking about general things, about how her two girls were doing and how life was out in Colorado. She didn't give me an indication that there was anything wrong. She seemed pretty happy," he said.

Shanann Watts had recently shared with family and friends that she was pregnant with their third child.

The case has focused attention on Colorado's lack of a law allowing homicide charges in the violent deaths of fetuses, which is the case in 12 states. Proposals to allow homicide charges in the violent deaths of fetuses in Colorado have been stymied by debate about how to avoid infringing on abortion rights.

Republican lawmakers last tried to change the law after a 2015 case in Boulder County. A woman named Dynel Lane was charged with attempted murder and unlawful termination of a pregnancy for cutting open a pregnant woman's belly and removing her unborn baby girl. Prosecutors said they could not charge Lane with murder because a coroner found no evidence the infant lived outside the womb.

State law does allow a homicide charge if a fetus was alive outside the mother's body and then killed. State lawmakers in 2013 also allowed prosecutors to add extra felony charges against anyone who commits a crime that causes the death of a fetus.